Poly(aryl ether ketone)s (i.e., PAEKs) are a well known class of engineering polymers useful in various fields of endeavor. Poly(ether ether ketone) (PEEK) and poly(ether ketone) (PEK) are the most common PAEKs. PEK and PEEK are high-strength, radiation-resistant engineering plastics whose structures combine both ether and ketone groups. Both are thermally stable and highly resistant to chemicals. Generally, PAEKs are prepared by aromatic nucleophilic substitution. For example, p-hydroquinone can be used as a nucleophilic component which is deprotonated with a base such as NaOH, Na2CO3, K2CO3, or a combination of Na2CO3 and K2CO3. The resultant phenolate may then react with, e.g., an aromatic dihalocompound, in particular a dihalobenzophenone such as 4,4′-difluorobenzophenone to form a PAEK, e.g. PEEK, via nucleophilic substitution, with the halogen atoms of the dihalobenzophenone acting as leaving groups. Certain dinucleophiles other than p-hydroquinone commonly used as monomers in the synthesis of PAEKs are bisphenols such as 4,4′-dihydroxybenzophenone, 4,4′-biphenol, 1,4-bis(p-hydroxybenzoyl)benzene, 1,3-bis(p-hydroxybenzoyl)benzene, . . . . Aromatic trinucleophiles, aromatic poly(>3)-nucleophiles, aromatic trihalocompounds, aromatic poly(>3)halocompounds, and mixtures thereof can also be used, generally in addition to the aromatic dinucleophile and the aromatic dihalocompound, when a branched or cross-linked PAEK is to be synthesized.
Often, such PAEK reactions are carried out in a solvent; the solvent may be, or may contain, diphenylsulfone. Additionally, the reaction is often, but not always, carried out using a cosolvent which forms an azeotrope with water, to help the removal of water from the reaction mixture, such as p-xylene.
To the best of the inventor's knowledge, the effect of sodium carbonate particle size on the characteristics of a PAEK produced therewith has not been thoroughly or systematically investigated.
For example, U.S. Pat. No. 4,320,224 (Rose et al.) describes the preparation of PEEK by reacting 4,4′-difluorobenzophenone with p-hydroquinone in the presence of at least one alkali metal carbonate or bicarbonate. In particular, Rose's Example 3 (which was submitted for comparison purposes) describes a polymerization process for making PEEK using a certain anhydrous sodium carbonate as sole alkali metal carbonate or bicarbonate; the PEEK made accordingly suffered from a low IV (equal to 0.48) and a rather dark color (absorbance of 0.20). Precisely, Rose teaches, notably on col. 7, 1. 46-50, that the formation of low molecular weight, dark-colored, brittle PEEK, results from the use of sodium carbonate or bicarbonate alone, and proposes, as sole remedies, to use instead a higher alkali metal carbonate or bicarbonate (such as K2CO3) either alone, or in admixture with Na2CO3. Unfortunately, Rose's proposed remedies are generally not as suitable as desired; indeed, the use of K2CO3 or another higher alkali metal has also some negative influence on polymer properties (resulting in gels and discolored polymer), as described in ICI Patent application EP001879, and in Zhuo N., Yubin Z., Zhongwen W., Xinyi T., Polymer Materials Science and Engineering, 1989, N 3, P 64-68. These ones are completely overlooked by Rose. Besides, Rose provides no information on the particle size distribution of the sodium carbonate of Example 3, except that that has been sieved through a 500 μm sieve; as a matter of fact, based on the low IV and dark color of the resulting PEEK, it can be concluded a posteriori, accounting for the Applicant's present contribution, that the sodium carbonate used by Rose had very likely a D90 well above 250 μm. More precisely, considering that that the most broadly available sodium carbonates are by far dense sodium carbonates (of which the D50 are typically of about 400 μm), it is very likely that Rose's sodium carbonate is a dense sodium carbonate that was sieved through a 500 μm sieve, and the sieving operation that was operated, did obviously not eliminate the big amount of particles having a diameter of 400 μm up to less than 500 μm which were contained in the dense sodium carbonate. Finally, it is noted that Rose does not provide any information on the possible importance that the particle size distribution of the alkali metal carbonate or bicarbonate may have on the PEEK polymerization process and polymer properties.
U.S. Pat. No. 4,636,557 is similar, using a combination of, e.g., sodium carbonate and calcium carbonate in the preparation of a PAEK and indicating that “the particle size of the carbonates used according to the invention is not in itself critical, but they are preferably used in a finely ground state and mostly have particle sizes smaller than 0.3 mm. The particle sizes are preferably between 1 and 250 μm”. Although it is not clear exactly what carbonate particle sizes were used in the several Examples of U.S. Pat. No. 4,636,557, Example 1 (using potassium carbonate) indicates that the particles were “ground to a particle size of less than 0.3 mm”.
U.S. Pat. No. 5,081,214 describes a process for the preparation of an aromatic polyether employing a mixture of sodium carbonate and sodium hydrogen carbonate. The reference states that the advantages achieved according to the invention are not dependent on the particle size of the carbonate compounds used, and further indicates that using a mixture of “coarse particle” soda having a particle size from 200 μm to 800 μm and sodium bicarbonate can help prevent unwanted dust formation during filling of the reaction vessel. The Examples of U.S. Pat. No. 5,081,214 use such “coarse particle” sodium carbonate while Comparative Examples use sodium carbonate having a particle size of 80 μm.
Finally, both U.S. Pat. No. 4,868,273 and U.S. Pat. No. 5,194,561 relate to the preparation of polyethers that can or must contain —SO2— linking groups in the presence of sodium carbonate. In U.S. Pat. No. 4,868,273 the sodium carbonate is desirably used in a finely divided form in order to avoid a product with a lower inherent viscosity (IV). For example, using sodium carbonate particles all below 0.261 mm a polymer product with an IV of over 0.7 was obtained, whereas with at least 50% by weight of the particles over 0.376 mm the IV of the product obtained was less than 0.7.
U.S. Pat. No. 5,194,561 describes a process for the preparation of an aromatic polyether in which metal carbonates including sodium carbonate are used in the form of finely ground salts. U.S. '561 teaches that polyether sulfone synthesis can proceed satisfactorily using sodium carbonate having a D90 value of about 50 μm; on the other hand, according to U.S. '561, polyether ketone synthesis would require a more finely ground material, with the preference being given to D9 values of below 30 μm, in particular below 20 μm. The expressed requirements in terms of particle size would result from differences of reactivity of the monomers involved in the polycondensation reactions.
PAEKs are known for their exceptional balance of technical properties, namely high melting point, good thermal stability, high stiffness and strength, good toughness and really excellent chemical resistance. Therefore, PAEKs have potential for a wide variety of uses, and their favorable properties class them with the best of the engineering polymers. However, PAEKs currently available to the trade suffer from certain disadvantages.
PAEKs currently available to the trade have an inherent yellow to dark grey color as formed, which limits their use in certain specific applications where lighter colors are needed. PAEKs having an improved, lighter color could find wider acceptance for many applications where color is a concern. Lower color PAEKs are thus clearly needed by the art and would represent a significant improvement over the PAEKs currently available to the trade.
Moreover, PAEKs are very good candidates for medical applications. For these ones, the presence in the PAEKs of residues of toxic compounds like p-xylene, which, as above explained, is otherwise helpful for removing the water from the reaction mixture, should desirably be avoided or at least reduced to a minimum.
In addition, there is also a need for PAEK featuring an improved processability.
These and other needs are met by certain embodiments of the present invention.